1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor thin film, and particularly to a method of manufacturing a thin film transistor (TFT) using a crystalline film containing silicon.
Incidentally, the term "semiconductor device" used in the present specification includes all devices functioning by using a semiconductor, and not only a single element such as a TFT, but also an electro-optical device and an electronic device equipped therewith are also included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique of constituting a semiconductor circuit by forming TFTs on a glass substrate and the like, has been rapidly developed. As such a semiconductor circuit, an electro-optical device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. Moreover, a system-onpanel with additional built-in logic circuits of a memory circuit, a clock generating circuit and the like has also been developed.
Since such a driver circuit and a logic circuit are required to be operated at high speed, it is not suitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, under the present circumstances, a TFT using a crystalline silicon film (polysilicon film) as an active layer has become the main stream.
In general, the crystalline silicon film is obtained by forming an amorphous silicon film on a glass substrate or a quartz substrate and crystallizing the amorphous film by irradiation of a laser beam or heating.
Since the substrate is hardly heated when the crystalline silicon film is obtained by the irradiation of a laser beam, the glass substrate can be used as the substrate. However, the crystallinity of the obtained crystalline silicon film is not so good. The characteristics of a TFT obtained by using the crystalline silicon film also becomes unsatisfactory.
On the other hand, the method of heating has a problem that necessary crystallinity can not be obtained by a heat treatment at such a temperature that the glass substrate can withstand.
There is also a method in which a quartz substrate is used and a crystalline silicon film is obtained by a heat treatment at such a high temperature as 900.degree. C. or more (a silicon film obtained by this method is especially called a high temperature polysilicon).
However, according to this method, the precipitation of grain boundaries is remarkable, and by this influence, the electrical characteristics of the obtained semiconductor device is not satisfactory.
The present inventors disclose a technique for obtaining a crystalline silicon film on the glass substrate in Japanese Patent Unexamined Publication Nos. 7-321337 and 8-78329, the disclosure thereof being incorporated herein by reference. In the technique disclosed in the publications, a catalytic element for promoting crystallization is selectively added into an amorphous silicon film, and by carrying out a heat treatment, a crystalline silicon film extending from the starting point of the added region is formed.
This technique can lower the crystallization temperature of the amorphous silicon film by the action of the catalytic element drastically by 50 to 100.degree. C., and the time required for crystallization can also be reduced to 1/5 to 1/10. Since the crystallization of the silicon film progresses in a lateral direction substantially parallel to the surface of the substrate, the present inventors call this crystallized region a lateral growth region.
Since the catalytic element is not directly added in the lateral growth region, the lateral region has a feature that the amount of the catalytic element remaining in the film is small compared with the case directly added. For example, although a region in which the catalytic element is directly added, includes the catalytic element in the order of 10.sup.19, the lateral growth region includes the catalytic element in the order of 10.sup.18 which is smaller than the former value by one figure.
As the above-mentioned catalytic element, a metal element such as nickel, cobalt and tin is used. Since such a metal element forms a deep level in a silicon film to capture a carrier, there is a fear that the metal element has a bad influence to the electrical characteristics and reliability of a TFT. This problem is not exceptional even for the above-mentioned lateral growth region.